Andrew G. Gerber

Numerical Estimation of Fluidelastic Instability in Tube Arrays

This study investigates unsteady flow in tube bundles and fluid forces, which can lead to large tube vibration amplitudes, in particular, amplitudes associated with fluidelastic instability (FEI). The fluidelastic forces are approximated by the coupling of the unsteady flow model (UFM) with computational fluid dynamics (CFD). The CFD model employed solves the Reynolds averaged Navier-Stokes equations for unsteady turbulent flow and is cast in an arbitrary Lagrangian-Eulerian form to handle any motion associated with tubes. The CFD solution provides time domain forces that are used to calculate added damping and stiffness coefficients employed with the UFM. The investigation demonstrates that the UFM utilized in conjunction with CFD is a viable approach for calculating the stability map for a given tube array. The FEI was predicted for in-line square and normal triangle tube arrays over a mass damping parameter range of 0.1-100. The effect of the P/d ratio and the Reynolds number on the FEI threshold was also investigated.

Collaborative CFD Exercise for a Submarine in a Steady Turn

The application of viscous-flow solvers to calculate the forces on ship hulls in oblique motion has been studied for a long time. However, only a few researchers have published work in which the flow around ships in steady turns was studied in detail. To predict ship manoeuvres, an accurate prediction of the loads due to rotational motion is also required. In a collaborative CFD exercise, the Submarine Hydrodynamics Working Group (SHWG) performed calculations on the bare hull DARPA SUBOFF submarine to investigate the capability of RANS viscous-flow solvers to predict the flow field around the hull and the forces and moments for several steady turns. In the study, different commercial as well as bespoke flow solvers were used, combined with different turbulence models and grid topologies. The work is part of a larger study aiming to improve the knowledge and understanding of underwater vehicle hydrodynamics. In this paper, the results of the exercise will be presented. For several cases, verification studies are done to estimate the uncertainties in the results. Flow fields predicted by the different members of the SHWG are compared and the influence of the turbulence model will be discussed. Additionally, the computed forces and moments as a function of the drift angle during the steady turns will be validated. It will be demonstrated that using sufficiently fine grids and advanced turbulence models without the use of wall functions will lead to accurate prediction of both the flow field and loads on the hull.© 2012 ASME

A Critical Review of Classical Force Estimation Methods for Streamlined Underwater Vehicles Using Experimental and CFD Data

Classical hydrodynamic force estimation methods are widely used by industrial designers of underwater vehicles for whom captive model experiments and CFD based simulations are uneconomical. They are also used in the preliminary design of submarines and when real time submarine simulations are required. These methods poorly estimate the contribution of the hull to the forces, especially at moderate to high incidence angles. This paper critically reviews the classical hull force estimation methods developed by Munk, Allen, Perkins and Jorgensen, and Sarpkaya. It compares the methods with experimentally validated CFD predictions of a streamlined body at incidence angles up to 30 degrees and for Reynolds numbers from 2.3 to 230 million. The comparison shows that inadequately modeled flow separation and leeside body vortices explain the poor force and moment predictions. This is partly due, at least, to the lack of a streamlined tail on the truncated missile shapes for which the estimation methods were developed.Copyright

Benchmarking of a Massively Parallel Hybrid CFD Solver for Ocean Applications

This paper presents progress on the development of a CFD program called EXN/Aero specifically designed to exploit performance gains from new hybrid multicore-manycore computer architectures. The hybrid multicore-manycore design is outlined along with performance and validation testing on an underwater vehicle and unsteady vortex shedding applications. It is shown that by revisiting CFD code design with a view to a number of important trends in the high performance computing industry, significant order of magnitude gains in computational power can be achieved.Copyright

Examination of the Flow Separation Characteristics Around a Streamlined Axisymmetric Shape

Using computational fluid dynamics (CFD), a study was conducted to predict the hydrodynamic forces and moments on an axisymmetric body over a range of yaw angles and Reynolds numbers. Computational results for hydrodynamic forces and moments show good agreement with experimental data, being within the experimental uncertainty range at most yaw angles. Deviations outside of the uncertainty range occurred for the lateral (Y) force values at yaw angles greater than 15 degrees. The development of the after-body vortex shows good agreement with experimental observation. Primary and secondary separation points and shear stress streamline behaviour are also compared with experiment data at a yaw angle of 24 degrees. Results are discussed with a view to identifying flow features critical to the development of new force estimation methods. The after-body vortex, at increasing yaw angles, influences the overall force and moment predictions through a complex interaction between the transport of after-body vorticity and the detachment/reattachment locations of the boundary layer. Adequate modeling of this after-body region is increasingly important at high yaw angles. One of the most important features that influences the overall forces and moments is the circumferential position of shear layer detachment and reattachment, which have a direct impact on the pressure distribution along the body.Copyright

Acceleration of unsteady hydrodynamic simulations using the parareal algorithm

Abstract The parareal algorithm is used to obtain temporal parallelization added to the parallelism obtained from the conventional spatial domain decomposition techniques for hydrodynamic problems. Parareal solution becomes unstable at high Reynolds numbers where the non-linear convection term in the Navier–Stokes equations becomes much larger than the diffusion term. A new framework that allows using parareal for unsteady high Reynolds number hydrodynamic problems is proposed where parareal coarse and fine time integration operators are incorporated with coarse and fine spatial grids respectively and RANS or DES turbulence models are employed with a blended filter that can be tuned for the stability of the method. This framework is composed of three solution stages where parareal serves as a transitional stage that bridges a coarse grid solution to a fine grid one. While in lower Reynolds number problems parareal solution can serve as a final solution, in higher Reynolds number problems with high degree of non-linearity parareal provides a shorter path to the final solution. Anticipating a parareal stage in a transitional sense allows a looser convergence requirement which leads to high speedup gains in that stage. On the other hand improved initial values at the beginning of the last stage yields a shorter final fine stage solution. A windowing technique is employed in this methodology to further control the parareal instabilities by keeping the parareal corrections smaller while still being able to cover an arbitrary simulation time with given computational resources. Application of this methodology has been illustrated with a fully turbulent vortex shedding from a cylinder and a flow from the Grand Passage tidal zone in the Bay of Fundy. It is concluded that a tuned turbulence model may sufficiently stabilize the parareal methodology for many practical problems such that it becomes applicable in the initialization process if not accurate enough as a final solution. MPI and OpenMP programming paradigms are used for temporal parallelism introduced by parareal and data parallelism obtained via spatial domain decomposition methods respectively. Also all computational tasks are accelerated using CUDA compatible GPGPUs.

Algebraic multigrid employing mixed structured–unstructured data on manycore hardware

Abstract This paper outlines investigations in computing performance when deploying a Computational Fluid Dynamics solution on CPU and GPU resources. Critical to the performance is an algebraic multigrid solver that concurrently utilizes mixed structured and unstructured data for solution. Software organization for manycore computing and data storage patterns for efficient memory access is described in detail along with performance testing on practical flow cases. It is shown that structured data blocks of sizes greater than 1 million, are solved more than 25× faster on a GPU compared to when solved using a single CPU thread. On the other hand unstructured data blocks does not reach more than 10× speedup for the same comparison. Consequently maximizing use of structured data blocks in a mixed data configuration allows a more efficient utilization of GPU acceleration while still benefiting from flexibility of unstructured blocks for grid generation purposes. Speedup obtained for mixed data block problems varies depending on the block configurations where an average 3× speedup is reported for a (76% structured, 24% unstructured) submarine incident flow problem in comparison with a same fully unstructured problem.

A Computational Study of Sprays Produced by Rotary Cage Atomizers

This article describes a computational study of poly-dispersed droplet spray plumes produced by Micronair AU4000 and AU6539 rotary cage atomizers that are commonly used in agriculture and forestry management. It combines experimental measurements of the droplet size spectrum and computational fluid dynamics (CFD) methods to develop a comprehensive understanding of droplet dispersion and spatial segregation in the near wake of the atomizer. The results will assist the development of improved measurement methodologies of pesticide droplet spectra. A parametric study of the Micronair AU4000 atomizer examines the effects of air speed, atomizer speed, liquid flow rate, and power source on spatial segregation of the plume. The RANS model uses a combined Lagrangian (droplet phase) and Eulerian (gas phase) procedure and includes the sprayer with blades and a large portion of the wind tunnel geometry. The computational results are compared to full-scale experimental measurements of pressure, gas phase velocity, turbulence intensity, and droplet size spectra measured using phase Doppler interferometry (PDI).

CFD Predictions of Efficiency for Non-Equilibrium Steam 2D Cascades

Most non-equilibrium wet steam CFD analyses in the open literature have concentrated on predicting blade pressure loadings, with very few studies emphasizing turbine efficiencies. One of the few exceptions is the work of Gerber et al. [1]. In light of this, in this paper we present CFD predictions of isokinetic efficiency and Markov Loss coefficients and comparisons with measurements for the 2D cascades of White et al. [2] and Bakhtar et al. [3, 4]. Predictions were obtained using an Eulerian-Eulerian multiphase formulation, which is an extension of General Electric’s proprietary CFD turbomachinery code, TACOMA. The formulation is optimized to capture the thermodynamic loss. There is no slip between the droplets and the surrounding vapor. Comparisons with other experimental quantities are also presented as needed to ensure that the non-equilibrium wet steam physics is accurately captured. Although the non-equilibrium models used cannot capture all the loss components present in actual flows, our efficiency predictions are much closer to experimental data than those of equilibrium simulations or the Baumann rule.

Numerical Estimation of Fluidelastic Instability in Staggered Tube Arrays

This study investigates the unsteady flow and the resulting fluidelastic forces in a tube bundle. Numerical simulations are presented for normal triangle tube arrays with pitch-to-diameter (P/d) ratios of 1.35, 1.75, and 2.5 utilizing a 2-dimensional model. In this model a single tube was forced to oscillate within an otherwise rigid array. Fluid forces acting on the oscillating tube and the surrounding tubes were estimated. The predicted forces were utilized to calculate fluid force coefficients for all tubes. The numerical model solves the Reynolds-Average Navier-Stokes (RANS) equations for unsteady turbulent flow, and is cast in an Arbitrary Lagrangian-Eulerian (ALE) form to handle mesh the motion associated with a moving boundary. The fluidelastic instability (FEI) was predicted for both single and fully flexible tube arrays over a mass damping parameter (MDP) range of 0.1 to 200. The effect of the P/d ratio and the Reynolds number on the FEI threshold was investigated in this work.Copyright